As it is perhaps well known, gentian violet dye is chemically
methylrosaniline chloride and has been used as both a
bacteriological and histological stain ...

http://dermatology.about.com/library...gentviolet.htm

Definition: Gentian Violet is a purple dye that is used to treat
vaginal yeast infections and thrush. The dye is "painted" on the
infection and kills the fungus. It can stain clothing.